A fluorophosphate optical glass has low dispersibility and abnormal dispersibility, and thus is a very useful glass as an optical material for correcting chromatic aberration at high-level. A fluorophosphate glass is produced by heating and melting a glass raw material, and rapidly quenching the obtained molten glass. Patent Documents 1 and 2 disclose typical techniques for producing a fluorophosphate glass by a melting method.
The method disclosed in Patent Document 1 is a method including melting an unvitrified raw material called a batch raw material, and the method disclosed in Patent Document 2 is a method including heating and melting a raw material called a cullet raw material that has been vitrified once.
Patent Document 1 discloses, as a preferable embodiment, a method for producing a fluorophosphate glass, which includes the steps of: melting a batch raw material in a melting container; feeding and defoaming the obtained molten glass to and in a refining tank; and further feeding and stirring the defoamed molten glass to and in an operation tank to homogenize.
Patent Document 2 discloses, as a preferable embodiment, a method for producing a fluorophosphate glass, which includes the steps of: introducing a cullet raw material in a melting container, heating and melting to give a molten glass; refining the glass by raising the temperature of the glass; removing bubbles; and stirring and homogenizing.